Recently, a control device for a fuel cell system which supplies a hydrogen gas to a hydrogen electrode of a fuel cell stack and the air to an air electrode of the fuel cell stack, which electrochemically reacts oxygen in the air at air the electrode with hydrogen at the hydrogen electrode, and which thereby causes the fuel cell stack to generate power has been studied. Particularly, development of a control device for an automotive fuel cell system which supplies the power generated by the fuel cell stack to a driving motor that generates a vehicle running torque is underway.
As one example of the automotive fuel cell stack, a polymer electrolyte fuel cell (PEFC) stack is known. The PEFC stack is configured to provide a solid polymer membrane between a hydrogen electrode and an air electrode, and to enable the solid polymer membrane to function as a hydrogen ion conductor. A hydrogen gas is decomposed to hydrogen ions and electrons in the hydrogen electrode, whereas an oxygen gas, hydrogen ions, and electrons are chemically bonded together to generate water in the air electrode. At this time, the hydrogen ions travel from the hydrogen electrode to the air electrode via the solid polymer membrane. In order so that the hydrogen ions travel via the solid polymer membrane, the solid polymer membrane needs to contain water vapor. Due to this, the fuel cell system control device needs to humidify the solid polymer membrane, and keep the solid polymer membrane humid. To this end, a technique for humidifying the hydrogen gas to be supplied to the fuel cell stack using a humidifier, and for supplying the resultant hydrogen gas to the hydrogen electrode is proposed.
As en effective technique for humidifying the solid polymer membrane, a hydrogen cycling technique for recirculating the hydrogen gas which is not used by the fuel cell stack but discharged therefrom to the fuel cell stack so as to be recycled by the fuel cell stack is known. In the fuel cell system which adopts the hydrogen cycling, the hydrogen gas is supplied to the hydrogen electrode by an amount slightly larger than a required hydrogen amount to generate power consumed by a load connected to an outside of the fuel cell stack, unused hydrogen gas is discharged from an outlet of the hydrogen electrode, and this exhaust hydrogen gas (hereinafter, “cyclic hydrogen”) is returned again to an inlet port of the hydrogen electrode. At this moment, the cyclic hydrogen discharged from the fuel cell stack contains much water vapor. Therefore, the cyclic hydrogen is mixed with dry hydrogen supplied from a hydrogen tank, a hydrogen mixture is supplied to the hydrogen electrode, and the hydrogen to be supplied to the hydrogen electrode is thereby humidified.
As can be seen, not only a hydrogen flow necessary for power generation but also an excessive hydrogen flow for humidifying the solid polymer membrane pass through the hydrogen electrode of the fuel cell stack. That is, by supplying a hydrogen flow more than the hydrogen flow necessary for power generation to the hydrogen electrode, all cells that constitute the fuel cell stack are enabled to efficiently generate power.
On the other hand, if only a hydrogen flow corresponding to a required power generation amount is supplied to the hydrogen electrode, then there is a probability that hydrogen does not efficiently reach the cells near the outlet port of the hydrogen electrode, and that the power generation efficiency of the fuel cell stack is deteriorated. Similarly, not an oxygen flow corresponding to the required power generation amount but an oxygen flow slightly larger than the oxygen flow corresponding to the required power generation amount is supplied to the air electrode. Namely, a material stoichiometric ratio that indicates a ratio of a consumed gas amount to a supplied gas amount is “1” when only the hydrogen flow or the oxygen flow corresponding to the required power generation amount is supplied. From viewpoints of humidification and power generation efficiency, the material stoichiometric ratio is normally set higher than “1”.
Nevertheless, even if the material stoichiometric ratio is optimum in a system design phase, it is not always optimum for an operating state of the fuel cell stack. Due to this, the material stoichiometric ratio is set higher so as to somewhat include a margin ratio, thereby setting supply amounts of hydrogen and oxygen to the fuel cell stack to be larger. As a result, materials such as hydrogen are disadvantageously wasted.
To eliminate waste of the materials, therefore, the material flow is reduced so that the material stoichiometric ratio is closer to “1”. If so, however, the supply amounts of hydrogen and oxygen need to be changed to be sensitive to a change in the operating state (a temperature, a humidity, a material distribution, and the like) of the fuel cell stack, thereby losing robustness. As a result, even with a slight change in the operating state, generated voltage is reduced to a lower limit or lower.
To prevent this disadvantage, fuel cell systems intended to appropriately control the material flow are disclosed in Japanese Patent Application Laid-Open Publication No. 2002-289235, No. 2000-208161, and No. 2002-164068. Each of the fuel cell systems disclosed therein controls the material flow so that a change in a cell voltage of a fuel cell stack falls within an allowable range, controls the material flow so that a generated voltage by the fuel cell stack is equal to or higher than a lower limit, and increases the material flow when the generated voltage of the fuel cell stack falls.